Fix warning about initialization order in BlitGL.
Fix warning about initialization order in BlitGL.

BUG=angleproject:1113

Change-Id: I808d521ffc257c2f45badbbd462698f91def314f
Reviewed-on: https://chromium-review.googlesource.com/295830
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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